This invention relates generally to methods and apparatus for computed tomographic (CT) imaging, and more specifically to methods and apparatus for acquiring and reconstructing helically scanned, medical CT images using a multi-slice CT imaging system.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. In a helical scan, a table on which the object is resting moves so that the object itself moves though the imaging plane while it is being scanned. A multi-slice CT imaging system has a plurality of parallel detector rows configured to acquire attenuation measurements corresponding to one or more two-dimensional image slices of an object. The number of image slices and the thicknesses represented by the slices is dependent upon how (and whether) attenuation measurements from the parallel detector rows are combined.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Helical reconstruction algorithms for multi-slice CT have been a focus for many studies. In one known CT imaging system, a reconstruction algorithm is implemented for two special helical pitches: 3:1 and 6:1. This algorithm utilizes two conjugate samples from different detector rows to estimate projection samples at a reconstruction plane using linear interpolation. Although this method performs satisfactorily in many cases, it has a number of shortcomings. First, the sampling pattern is not always optimum because only two samples on either side of the plane of reconstruction are selected. For example, samples that are closer to the reconstruction plane but located on the same side of the plane will not be utilized. Second, a 3:1 helical pitch is non-optimal for projection sampling, because the first and last detector rows measure identical ray paths, reducing the amount of non-redundant information that is acquired. In fact, in one known helical reconstruction implementation, measured projections (after calibration) of these two rows are summed first before reconstruction takes place. Third, sharp structures in the original object (along a z-axis) are suppressed and degraded slice sensitivity profiles are obtained because linear interpolation suppresses high frequency information in the sampled data.
It would therefore be desirable to provide methods and apparatus for helical reconstruction in multi-slice CT imaging systems that overcome the above-described shortcomings of known image reconstruction systems.